A basic fuel cell comprises an anode electrode spaced apart from a cathode electrode with an electrolyte disposed between the two electrodes; each electrode includes a catalyst layer on the electrolyte side thereof. On the non-electrolyte side of the anode electrode is a reactant gas chamber for carrying a fuel, and on the non-electrolyte side of the cathode electrode is a reactant gas chamber for carrying an oxidant. The electrodes are constructed so that the gas diffuses therethrough and comes into contact with the electrolyte in the catalyst layer thereby causing a well-known electrochemical reaction whereby hydrogen ions and electrons are produced at the anode. The electrons travel from the anode electrode through an external circuit to the cathode electrode where they react with oxygen to produce heat and water. This flow of electrons is the electric current produced by the cell.
In a proton exchange membrane (PEM) fuel cell power plant, a number of fuel cells are connected electrically in series, forming a cell stack assembly (CSA). The cells of the CSA are sandwiched between end plates bolted together to hold the cells in tight contact with one another.
Cell stack assemblies that utilize gaseous reactants typically have opposed pairs of external manifolds which distribute the reactant gases to the cells in the stack, and gather reactant exhaust gases from the cells in the stack, as disclosed for example in commonly owned U.S. patent application Ser. No. 09/920,914, (PCT publication number US 2003-0027029) Typically, the pairs comprise a fuel inlet/exit manifold opposite a fuel turn manifold and an air inlet manifold opposite an air outlet manifold. Each manifold must be sealed to the cell stack assembly to prevent leakage of the reactant gases into the ambient environment. A manifold retention system may include load cables to provide a manifold-to-CSA sealing force.
Generally, the dimensional tolerances of the individual cells and the position tolerances of the cells within the cell stack assembly result in cell edge misalignment, known as an uneven “skyline”, within the PEM cell stack assembly.
Referring to FIG. 1, a typical prior art PEM fuel cell manifold-to-CSA seal arrangement, disclosed in commonly owned U.S. patent application Ser. No. 09/882,750, (PCT publication number US 2001-0055708-A1), is illustrated. Fuel cell elements 8, which together form an uneven skyline, are sandwiched between endplates 9 bolted together so as to hold the individual cells 8 in tight contact with one another. A reactant gas manifold 10 (either oxygen containing oxidizing gas or hydrogen containing fuel gas) is positioned adjacent the cell elements 8 so as to provide a flow of reactant gas to and from the CSA 35. The prior art seal system 13 includes one or more filler layers 22 of silicone liquid rubber applied to the skyline of the stack surface to form a flat relatively smooth surface above the elements 8, overlapping the sealing surfaces 17 on the endplates 9. A molded silicone rubber gasket 15 is bonded to a contact surface 16 of the manifold 10. The gasket 15 is used in conjunction with a flat rubber strip 20, typically a molded precast silicone rubber strip, interposed between the silicone rubber filler layer 22 and the gasket 15.
When subject to a sealing force between the manifold 10 and CSA 35, the layers of sealing materials, 20, 22 and 15, compress to form a tight seal. Even when compressed, however, the intervening layers of sealing materials result in a necessary clearance 24 between contact surfaces 16 and 17 of the manifold 10 and endplates 9 respectively.
Although well suited for stationary fuel cell power plant applications, such manifold-to-CSA seals disclosed in the prior art have certain limitations when used in automobiles or other vehicles subject to the stresses of acceleration and vibration. In particular, the seals may experience compressive creep over time, which reduces the sealing force exerted by the load cables, and can result in reactant leakage and slipping of the seals. Moreover, the rubber strip 20, between the gasket 15 and the silicone rubber filler 22 transfers the compressive forces on the seal to the cell stack assembly non-uniformly and may result in cracked cell components 8 at the high spots in the skyline.